This invention relates to an improved process for the preparation of amino methyl cyclo alkane acetic acids. This invention particularly relates to an improved process for the preparation of gabapentin (which is chemically known as 1-aminomethyl-1-cyclohexaneacetic acid), which is a very well known agent useful for the treatment of epilepsy and other cerebral disorders. In the chemical series of 1-amino methyl cyclo alkane-1-acetic acids, Gabapentin, which is 1-amino methyl cyclo hexane-1-acetic acid has been developed as a drug having anti convulsive properties.
Gabapentin has the formula I shown below

U.S. Pat. No. 4,024,175 & 4,087,544 and DE Pat. No 2460891 disclose this compound, process of its preparation and its uses.
The above patents describe various processes for the preparation of Gabapentin and similar compounds of the general formula 2 given below

Wherein R1 is a hydrogen atom or a lower alkyl radical and n is an integer with a value of 4 to 6 and their pharmaceutically acceptable salts.
The processes disclosed in these patents are based on known methods used for the preparation of primary amines. Specifically, they involve Curtius reaction of cycloalkane diacetic acid monoesters, Hoffmann reaction of cycloalkane diacetic acid monoamides or Lossen Rearrangement of 1-carboxymethylcycloalkane acetohydroxamic acid sulphonate esters.
In a variation of the Lossen Rearrangement, the process can be carried out on the O-sulphonyloxycycloalkane-1,1-diacetic (N-hydroxy)imide (U.S. Pat. No. 4,152,326, & Can patent no 1085420). These procedures go through an isocyanate or urethane that can be converted into the desired 1-aminomethyl-cycloalkane-1-acetic acid by acidic or basic hydrolysis. The aminoacid hydrochloride is isolated from the hydrolysate by evaporation of water.
In particular, in U.S. Pat. Nos. 4,024,175 and 4,087,544 and DE Pat. No. 2460891 monomethyl cyclohexane-1-acetic acid was transformed to the azide which was decomposed (Curtius reaction) in boiling toluene. The resultant isocyanate was hydrolysed with aqueous hydrochloric acid. The resultant solution was evaporated to dryness to give 1-aminomethylcyclohexane-1-acetic acid hydrochloride, which was converted to gabapentin with a basic ion-exchange resin.
In the same patents, 1,1-cyclopentane diacetic acid monoamide was treated with aqueous sodium hypobromite at −10 deg and the solution then heated at 60 deg for 2 hours. It was then acidified with 12 N hydrochloric acid and evaporated in vacuum. The residue was extracted with ethanol and the ethanol solution evaporated to give 1-amino methyl cyclopentane-1-acetic acid hydrochloride from which the free aminoacid was obtained by passage through a basic ion exchange resin.
In U.S. Pat. No. 4,152,326, N-(p-toluenesulphonyloxy)-1,1-cyclohexanediaceticacid imide was heated with 10% aqueous sodium hydroxide solution (Lossen Rearrangement) at 100 deg and the resultant solution acidified with concentrated hydrochloric acid and evaporated to dryness. The residue was digested with ethanol and filtered and the filtrate evaporated in vacuum to give gabapentin benzenesulphonate. Treatment of this material with the basic ion exchanger, IR-45, in the —OH form gave gabapentin.
In the ensuing years, there have been patents involving other routes which involve the hydrolysis of 2-azaspiro (4,5) decan-3-one of the formula 3, known conveniently as gabalactam, first isolated by Sircar (J. Ind. Chem. Soc., 1928, 5, 549; chem. Abstracts, 1929, 23, 818) with 1:1 hydrochlorid acid

(U.S. Pat. Nos. 5,091,567, 5,068,413, EP 414263 WO patent application no 9914184A1) to afford gabapentin as the hydrochloride salt.
Thus in U.S. Pat. No. 5,068,413 and EP patent no 414263,A2, gabalactam is mixed with 1:1 hydrochloric acid and boiled under reflux at 108 degree C. for 6 hrs, cooled and diluted with water. The mixture is extracted with methylene chloride to remove un-dissolved lactam. The aqueous solution is evaporated to dryness in vacuum and the residue washed with acetone to give gabapentin hydrochloride as the insoluble part.
In U.S. Pat. No. 5,091,567, gabalactam is similarly hydrolysed with hydrochloric acid to give gabapentin hydrochloride.
In WO patent application no 9914184A1, the lactam of the formula 3 is hydrolyzed with a mixture of 6N hydrochloric acid and dioxane at reflux for 4 hours. The solution is evaporated to dryness and the residue crystallized from methanol-ethyl acetate-heptane to afford gabapentin hydrochloride.
In two other patents (U.S. Pat. Nos. 4,956,473 & 4,958,644), the lactam of the formula 3 having an extra carbethoxy groups has been synthesized and hydrolyzed with 1:1 hydrochloric acid with concomitant removal of the carbethoxy group to afford gabapentin hydrochloride.
In U.S. Pat. No. 4,958,044, a solution of (1-cyano cyclohexyl) malonic acid dimethyl ester in ethanol was hydrogenated at 10 bars of hydrogen pressure and 90 deg C. on 3 g Raney Nickel for 4.5 hours. The solution was filtered and the filtrate evaporated to give 2-aza-(4-methoxy-carbonyl)spiro (4,5) decan-3-one (carbethoxy gaba lactarn). This was mixed with 20% hydrochloric acid and stirred under reflux for 24 hours. The solution was evaporated to dryness and the residue worked up to give gabapentin hydrochloride.
Other methods to synthesize gabapentin directly without the intervention of gabalactam or gabapentin hydrochloride have also been described. In U.S. Pat. Nos. 5,095,148, 5,135,455, 5,136,091, 5,149,870 & Can patent no 203017, (1-cyano cyclohexyl) acetic acid benzyl ester was hydrogenated in methanol using 5% Rhodium on carbon catalyst at 10 bars of hydrogen pressure for 23 hours at room temperature. Filtration of the mixture, concentrating the filtrate and diluting with ethanol gave a 27% yield of gabapentin.
In U.S. Pat. Nos. 5,132,451, 5,319,135 & 6,294,690, 1-cyanocyclohexane acetic acid was hydrogenated in methanol at room temperature for 2 hours, using 15% Rhodium on carbon catalyst containing 1% palladium. The mixture was filtered and the filtrate concentrated. Addition of isopropanol and stirring at 0-5 deg for 24 hrs gave gabapentin. In EP Patent no 414262B1,1-cyano cyclohexene acetic acid was hydrogenated on Raney nickel to produce Gabapentin.
In U.S. Pat. No. 6,294,690, benzo nitrile was subjected to a Birch reduction with lithium and liquid ammonia and the reduction intermediates trapped with ethyl bromo acetate. The resultant product was hydrolysed to (1-cyano cyclo hexa-2,5-dienyl) acetic acid which was hydrogenated in methanolic ammonium hydroxide for 3.5 hrs at 50 deg C. and 50 psi hydrogen pressure and on 5% palladium charcoal catalyst. The mixture was filtered and the filtrate concentrated to give crude gabapentin.
WO application No 2000039074 describes synthesis of gabapentin by hydrogenation of 1-nitromethylcyclohexyl acetic acid benzyl or diphenylmethyl ester.
By far the most widely used procedure for the preparation of gabapentin appears to be the removal of HCl from its hydrochloride salt. This has been accomplished in various ways, all of which except for four processes use a basic ion exchange resin (U.S. Pat. Nos. 4,024,175, 4,894,476, 4,960,931 & 6,054,482; Canada patent no 1085420; EP patent no 340677, 414263, WO patent application Nos 9914184 & 0001660) wherein the hydrochloride was mostly dissolved in water or some times in water and an alcohol.
In U.S. Pat. No. 4,024,175, gabapentin was obtained from its hydrochloride by treatment with a basic ion exchanger and crystallization from ethanol-ether. No experimental details are given in this patent.
In U.S. Pat. Nos. 4,894,476, 4,960,931 & 6,054,482, EP patent no 340677 a solution of gabapentin hydrochloride in de ionized water was poured into a column of Amberlite IRA-68 in the OH form and the column eluted with de ionized water. The eluate was concentrated on a rotovap at about 29-31 degree C. in vacuum to slurry. The slurry was mixed with isopropanol and cooled to give gabapentin monohydrate.
In Canadian Patent no 1085420, gabapentin benzene sulphonate salt was converted to gabapentin by exchange on Amberlite IR 45.
In EP Patent no 414263, gabapentin hydrochloride was converted to gabapentin by de ionising with the ion exchange resin IRA 68.
In WO 0001660, a solution of gabapentin hydrochloride in water was passed over ReliteEXA10 resin and the eluate was concentrated under vacuum. The concentrate was treated with 2-methoxy ethanol and a mixture of water and 2-methoxy ethanol was distilled out. Isopropanol was added to the resultant suspension; the mixture was heated to 60 deg C. for 30 minutes and cooled. After 2 hours at −5 to −10 deg C., the precipitate was filtered to give gabapentin.
Among the exceptions, in one case de-ionisation has been carried out in methanol. Thus in EP Patent no 1174418A 1 and WO 200064857, deionisation of a solution of gabapentin hydrochloride in methanol was achieved by passing through a weakly basic ion exchange resin BAYER MP-2. The methanolic eluate was concentrated by low-pressure distillation below 30 deg C. to give a dense suspension which was dissolved in methanol-water at 65 deg C. cooled and treated with isopropanol to give pharmaceutical grade gabapentin.
In another process reported in WO patent application no 00/58268, an aqueous solution of gabapentin hydrochloride was neutralized with 1 M NaOH to a pH of 7.14 and subjected to diafiltration at about 22 deg C., using a nano filtration multiplayer composite membrane having high selectivity for organic compounds with molecular weight higher than 150 and low selectivity to inorganic mono valent ions. The resultant solution is concentrated under reduced pressure below 35 deg C. and gabapentin is precipitated by isopropanol and crystallized from methanol.
In another process described in U.S. Pat. No. 6,255,526 B1, gabapentin hydrochloride was suspended in ethyl acetate and stirred with tri-n-butylamine at 25 deg C. for 2 hours. The precipitated gabapentin was collected by filtration and stirred with methanol at 25 deg C. for 14 hours and filtered off.
In another process presented in WO patent application no 02/34709-gabapentin hydrochlorides, obtained as a solution in n-butanol was poured over strong cationic resin (IMAC HP 1/10). After washing the column with water, gabapentin was eluted with aqueous ammonia. The ammonium solution was evaporated below 40 deg to a thick residue, which was heated with methanol and then stirred with isopropanol. The mixture was filtered to give gabapentin.
It can be seen from the above prior art literature; the process for the preparation of gabapentin is to access gabapentin hydrochloride by a suitable method and then subjecting it to ion exchange treatment. This process leads to the formation of gabapentin, mostly an aqueous solution, which is then evaporated. This process has to be conducted at a low temperature of 25 to 40 deg C. and a high vacuum in the range of 1 to 2 torr as otherwise lactamisation results leading to contamination of the resulting product. Water, having a low vapour pressure, the process of evaporation will be tedious and time consuming. In addition the use of a high vacuum for such long lengths of time will consume much energy. Hence the process becomes cost-inefficient and user un-friendly and therefore may not be suitable for industrial applications.
U.S. Pat. No. 6,054,482 claims that only by using the ion exchange method, gabapentin hydrochloride will give the pure aminoacid with less than 0.5% of residual gabalactam and 20 ppm of chloride. Above these levels, the storage stability of gabapentin is adversely affected, with build up of toxic gabalactam to undesirable levels.
On the other hand, USP 2002/0061931, demonstrates that the presence of chloride ion above 20 ppm upto 100 ppm and of gabalactam upto 0.5% in samples of gabapentin obtained from its hydrochloride by the method outlined in U.S. Pat. No. 6,255,526 (for example suspension of the hydrochloride in ethyl acetate stirred with (tri-n-butylamine for 2 hours at 25 deg C. and filtered) have the desired stability.
Another important aspect to be considered while developing a process for the preparation of gabapentin from its hydrochloride is regarding the purity of Gabapentin which is to be used in the pharmaceutical applications/formulations containing it. This aspect, which is a recent development, is concerned with the stringent specifications proposed by the Pharmaceutical Forum. Some of the important specifications stipulated and which are relevant to the present invention, are the following:
1.Chloride contentNMT 100 ppm2.Gabalactam contentless than 0.1%3.Impurity with RF 0.5 relative to gabapentinless than 0.2%4.Any other individual impurityless than 0.1%5.Total impuritiesless than 0.5%,excluding theimpurity mentionedin item 3
The specifications of individual formulators are even more stringent with limits wherein the limitation of Gabalactam should be less than 0.05% and impurity with RF 0.5 relative to gabalapentin being—less than 0.1%.
Therefore the gabapentin which will be obtained by any process should meet the above stringent requirements as otherwise it will not be useful for pharmaceutical applications. The method at the same time must be capable of affording pure gabapentin conforming to the stringent specifications of the Pharmaceutical Forum mentioned earlier.
Currently Gabapentin is a high selling drug (falling within top ten in the world market) as it can also be used for the treatment of deep neural pain, in addition to its known anti epileptic activity. Understanding clearly from the above described state of the art literature and taking into consideration the stringent pharmaceutical specifications it is felt that if a simple, inexpensive method is developed for the preparation of gabapentin from its salts, especially hydrochloride, avoiding low temperature evaporation of large volumes of solvents such as water or use of tertiary amines which are likely to contaminate the final products, it would lead to a process for preparing Gabapentin in commercial quantities to meet the increasing global demand.
Accordingly we took up research and development work towards development of an improved process for the preparation of gabapentin. Gabapentin so prepared meets the stringent pharmaceutical specifications mentioned earlier
Accordingly, the main objective of the present invention is to provide an improved process for the preparation of gabapentin overcoming the above-mentioned difficulties and to produce gabapentin meeting the stringent pharmaceutical specifications.
Another objective of the present invention is to provide an improved process for the preparation of gabapentin, which does not involve the costly ion exchange conversion of gabapentin hydrochloride, making the process simple and economical
Yet another objective of the present invention is to provide an improved process for the preparation gabapentin which results in high purity (over 99.5%) gabapentin
Still another objective of the present invention is to provide an improved process for the preparation of gabapentin which results in high yield (over 50%)
From the above mentioned prior art literature relating to the preparation of gabapentin it would be observed that an easy and simple method for the liberation of gabapentin from its hydrochloride salt would be to neutralize an aqueous solution of the latter with aqueous alkali or alkali earth hydroxide. This method, surprisingly, has not been attempted or reported in the literature. Such a method has not so far been attempted, perhaps, considering the fact that gabapentin, being an aminoacid, will be water soluble and cannot be precipitated even at the isoelectric point in desirable yields namely of more than 50%.
In the Indian patent No 186285, a process for the preparation of gabapentin has been disclosed which produces a substantially pure gabapentin. The process described is given below:
The process involves isolation of substantially pure 1-(aminomethyl) cyclohexaneacetic acid directly from an aqueous solution of its acid addition salt, The acid addition salt used is an addition product of 1-(aminomethyl)cyclohexaneacetic acid with a mineral acid selected from hydrochloric acid, sulphuric acid, phosphoric acid, nitric acid, or with an organic acid selected from C1 to C12 aliphatic carboxylic acid, C1 to C7 aliphatic sulphonic acid, aryl sulphonic acid and a polycarboxylic acid, comprising addition of a base to an aqueous solution of the 1-(aminomethyl)cyclohexaneacetic acid addition salt to adjust the pH between 6.7 to 8 to precipitate 1-(aminomethyl)cyclohexaneacetic acid, followed by washing the precipitated 1-(aminomethyl)cyclohexaneacetic acid with a water miscible organic solvent, and drying the precipitated 1-(aminomethyl)cyclohexaneacetic acid to obtain substantially pure 1-(aminomethyl)cyclohexaneacetic acid which contains less than 0.2% 2-azaspiro[4,5]decan-3-one.
The base employed in the process is generally an alkali or alkaline earth metal hydroxide or carbonate.
Specific examples of alkali and alkaline earth metal hydroxides and carbonates that may be used include NaOH, KOH, LiOH, CsOH, Mg(OH)2, Ca (OH)2, Ba(OH)2, Li2CO3, Na2CO3, K2CO3, Cs2CO3 or mixtures thereof. The preferred base is an alkali metal hydroxide. More preferably the base is sodium hydroxide.
During the treatment with hydroxide base in the process, the pH is adjusted from an acidic pH to a pH between 6.7 to 8, preferably between 7.2 to 7.8.
The adjustment of pH with hydroxide base is carried out at temperature between 0 to 50 deg C., preferably between 10 to 40 deg C. and more preferably between 15 to 25 deg C. After adjusting the pH the reaction mixture is allowed to stand for sufficient time ranging from 1 to 12 hour for efficient crystallization of 1-(aminomethyl)cyclohexaneacetic acid. The crystallized 1-(aminomethyl) cyclohexaneacetic acid is washed with a water-miscible organic solvent, preferably acetone and dried to obtain substantially pure anhydrous 1-(aminomethyl)cyclohexaneacetic acid.
When we tried to repeat the above-explained process with the help of details given in the experimental section (Example 2) of Indian patent no 186285, we found that gabapentin so prepared was not falling within the pharmaceutical specifications explained above. The gabapentin obtained by the above method was found to be containing un-acceptably large amount namely more than 3% vs less than 100 ppm of chlorides and total impurity levels of more than 1.5% as against 0.5% required by pharmacopia
Therefore we observed that the above-mentioned process seems to be not suitable for the preparation of gabapentin satisfying the stringent pharmaceutical specifications explained above.
Systematic and sustained investigations made by us with (i) various volumes of water for dissolving gabapentin hydrochloride, (ii) various strengths of neutralizing alkali, (iii) various temperatures of neutralization, (iv) various aging time of the precipitate, (v) various compositions of liquids for washing the filter cake and (vi) crystallization procedures resulted in our finding that an improved process for the preparation of gabapentin can be developed Gabapentin prepared by the process developed according to the process of the present invention, meets the above mentioned stringent pharmaceutical stipulations and also results in good yields (say 40 to 60%) with total impurity levels less than 0.5% and chloride contents less than 100 ppm as required by pharmacopia
Thus, according to the present invention, it is possible to prepare gabapentin, which will have the purity of 99.5%, yield of 40 to 60% and finally meeting all the above-mentioned stringent specifications for its use in the pharmaceutical field.
In other words we could achieve a result, which could not have been anticipated by the prior art knowledge.
Accordingly the present invention provides an improved process for the preparation of gabapentin of the formula 1
which comprises(i) preparing an aqueous solution of Gabapentin hydrochloride in water in the ratio of one part by weight of the former to 0.5 to 3 parts by weight of the later,(ii) preparing an aqueous solution of an alkali metal base in a concentration in the range of 40-50% w/w(iii) adding 0.08 to 0.3 parts by weight of the solution obtained in step (ii) to 1.5 to 4 parts by weight of the solution obtained in step (i) at a temperature in the range of 0 to 20 degree C.(iv) heating the resulting solution gradually to a temperature in the range of 50-90 degree C.(v) gradually cooling the resulting solution to a temperature in the range of 0 to 15 degree C. to obtain a precipitate,(vi) aging the precipitate for a period in the range of 0.5 hrs to 8 hrs at a temperature in the range of 0 to 15 degree C.(vii) Separating the precipitate from the mother liquor by conventional methods and(viii) recrystallising the precipitate from a mixture of IPA, Methanol & water to get Gabapentin of over 99.5% purity and a mother liquor
In a preferred embodiment of the invention the various steps may be performed as follows
The amount of gabapentin hydrochloride and water used in step (i) may preferably be 0.5 to 2.5 parts of water to 1 part of the Gabapentin hydrochloride and more preferably 1.5 to 2.5 parts of the water
The alkali used in step (ii) may preferably be sodium hydroxide or potassium hydroxide, more preferably sodium hydroxide. The solution used may be in a concentration in the range of 40-50% w/w more preferably in the concentration in the range of 45-50% w/w in water.
The temperature employed in step (iii) may be preferably 10-20 deg C. and more preferably 10-15 deg C.
The temperature employed in step (iv) used may preferably be 50-75 deg C. and more preferably 60-70 deg C. The gradual heating may be effected during a period of 1 to 3 hrs
The temperature employed in step (v) may be preferably 5-15 C. deg and most preferably 5-10 deg C. The said cooling may be effected gradually during a period of 1.5 to 3 hrs
The time employed for aging the precipitate in step (vi) may preferably be between 0.5 to 3 hrs and more preferably 0.5 to 1 hr.
The method of separation used in step (vii) may preferably be filtration, more preferably centrifugation].
In another preferred embodiment of the invention the solution of the gabapentin hydrochloride prepared is treated with charcoal and filtered through hyflobed to de-colourise the solution before basification.
The process of the present invention while avoiding the usage of liquid resins, aromatic amines and high-energy requirements, gives cycloalkane amino methyl acetic acids in pharmaceutically purer form. The process gives 1-amino-methyl-cyclohexane acetic acid in which the sum of all impurities determined by the pharmacopoeia method as described in USP-NF is not more than 0.5% and no unknown impurity more than 0.1%. The toxic gabalactam is also controlled to a limit of less than 0.1%. The chloride contents are substantially lower than (between 50-60 ppm) the prescribed pharmacoepial limits, (less than 100 ppm) while not compromising on the final yield of the material.
It is to be noted that the process defined above can be extended to other 1-aminomethylcycloalkane-1-acetic acids of the general formula 2
where ‘n’ represents an integer from 4-6.
This can be done from precursor lactams of the general formula 4,
where n has the meaning given above, through the intermediacy of the hydrochloride salts which can be neutralized with alkali or alkali earth hydroxide solutions.
The process of the present invention has been made more economical by utilizing the mother liquors resulting from the steps (vii) & (viii) of the process to prepare Gabalactam of the formula 3.
Accordingly the invention also provides a novel improved process for the preparation of Gabalactam of the formula 3 which comprises treating the mother liquors obtained in steps (vii) & (viii) of the above mentioned process with aq.sodium hydroxide in a concentration in the range of 5 to 20% at a temperature in the range of 80 to 100 degree C., recovering the gabalactam by extraction with organic solvents.
In a preferred embodiment the concentration of sodium hydroxide may range from 10- to 20%, the temperature may range from 80 to 85 deg C.
In yet another embodiment the recovery of gabalactam can be effected by extracting the reaction mixture with solvents such as toluene, ethylene dichloride, methylene dichloride or hexane preferably toluene.
The gabalactam, which is so prepared, can be used for the preparation of Gabapentin Hydrochloride, which is the starting material for the process preparation of gabapentin as explained above.
The details of the invention are given in the Examples given below which are provided only for illustrative purposes and therefore should not be construed to limit the scope of the invention